The estimated weights are relative, in the sense that any weights that are proportional are equally valid [1]}. Weighting reduces the ESS of the index trial. The approximate ESS after weighting is typically estimated as \(\left(\sum _i^n\hat{w}_i\right)^2/\sum _i^n\hat{w}_i^2\)  [2]}, [3]}. Low values of the ESS suggest that a few influential participants with disproportionate weights dominate the reweighted sample.
